Add `look_at` alternative documentation for RigidBody2D

**Your Godot version:** 4.1.1

**Issue description:**

RigidBody has helpful documentation on how to implement a workaround for the `look_at` method since this method does not work for RigidBody nodes. This documentation is linked below.

https://docs.godotengine.org/en/stable/tutorials/physics/rigid_body.html#the-look-at-method

Unfortunately, I needed a solution to get `look_at` functionality for RigidBody2D, but could not find a similar workaround in the docs. It would be helpful to have an alternative (probably simpler) implementation documented for 2D space.

**URL to the documentation page (if already existing):** https://docs.godotengine.org/en/stable/tutorials/physics/physics_introduction.html#using-rigidbody2d